Method of processing sucker rods and resulting article



Jan. 13, 1970 F, CURRENT 3,489,620

METHOD OF PROCESSING SUCKER RODS AND RESULTING ARTICLE Filed March 1, 1967 Affamey United States Patent US. Cl. 148-146 9 Claims ABSTRACT OF THE DISCLOSURE A sucker rod surface is induction-hardened to provide a martensitic case. The case is further compressed as by shot-peening. The resulting rod has increased resistance against failure through fatigue, yet successfully resists loss of tensile strength through hydrogen embrittlement.

This invention relates to an improved method of processing sucker rods and to the resulting article.

In the oil well pumping art, sucker rods are rods which extend down a well for transmitting reciprocating motion from a pumping unit at the surface to a subsurface pump near the bottom. The individual rods are connected end-to-end by internally threaded couplings, which engage externally threaded pins at the ends of the rods above and below. A sucker rod string is subjected to cyclic loads as it moves up and down, and such loads often lead to fatigue failure. Corrosive attack by fluids in the well or abrasion of the rods against the well tubing may hasten such failure.

Hydrogen sulfide is present in most oil wells, and causes hydrogen embrittlement of sucker rods. It is known that sensitivity of carbon steel to hydrogen embrittlement greatly increases as the steel is hardened. Hydrogen ernbrittlement lowers the ability of steel to withstand tensile loads. Hence those skilled in the art would not expect any benefit to result from casehardening a sucker rod to combat fatigue failure, since the harder surface would lead to premature failure for other reasons.

An object of the present invention is to provide an improved sucker rod and processing method which significantly increase resistance of the rod against fatigue failure, yet avoid increasing its susceptibility to failure from other causes (e.g. hydrogen embrittlement).

A further object is to accomplish the foregoing object, and thus prolong the rod life, at reasonable cost.

A more specific object is to provide an improved rod and method in which a critical portion of the rod surface is subjected to a series of treatments including inductionhardening and further compressing, as by shot-peening, whereby resistance against fatigue failure is markedly enhanced.

In the drawing:

FIGURE 1 is a side elevational view partly broken away of a portion of a sucker rod processed according to my invention;

FIGURE 2 is a similar view showing a rod of modified construction; and

FIGURE 3 is a similar view showing a rod of an other modified construction.

FIGURE 1 shows a sucker rod which includes an elongated body 10, and an enlarged elevator button 12 at the end of the body. Beyond the elevator button the rod includes in succession a wrench square 13, a pin shoulder 14, an undercut 15 and a threaded pin 16. FIGURE 2 shows a sucker rod which has a rounded or bulbous elevator button 12a, but otherwise is of similar configuration to FIGURE 1. FIGURE 3 shows a sucker rod, the wrench square 13a of which has rounded edges, but otherwise is of similar configuration to FIGURE 1. In

3,489,620 Patented Jan. 13, 1970 each instance the structure is duplicated at the opposite end. The sucker rod is of steel, the preferred analysis of which is as follows:

Percent C 0.15-0.50 Mn 0.30-1.75 P 0-0.07 S 0-0.3 5 Si 0.10-0.35 Ni 0 3 .50 Cr 0-1.25 Mo 0-0.3 0 B 00.005 V 0-0.05 Al 0-0.05

Balance iron and incidental impurities.

High sulfur is undesirable, since it lowers the resistance of the rod to corrosion. Surface decarburization of the rod should be avoided. The carbon content at the surface should be at least 0.15 percent to achieve a stress pattern, as hereinafter described, and to enhance corrosion resistance.

As in conventional practice, I form the rod in a forging operation. According to one procedure, I heat-treat the forged rod by austenitizing and cool it in air or quench it in liquid. I temper the rod, straighten it and remove scale from its surface. After processing the rod according to my invention, hereinafter described, I machine threads on the pins 16.

In accordance with my invention, I heat the body 10 of the rod by induction and quench it to produce a substantially martensitic case 17. I define a substantially martensitic case as a case which is free-of ferrite, but may contain a small amount of retained austenite. The depth of case should be about 3 to 15 percent of the rod diameter, or preferably about 5 to 8 percent. A case depth less than about 3 percent is inadequate, while a case depth greater than about 15 percent makes the rod too brittle. The case may extend the full length of the rod except for the threaded pins 16. However, with the form shown in FIGURE 1 I prefer to terminate the case at the plane where the elevator button 12 reaches its maximum diameter, or only slightly beyond with the elevator button 12a of FIGURE 2. If the case extends into the wrench square 13 of these forms, it is difficult to achieve uniformity and there is risk of cracking the rod when a coupling is tightened on its threads. Also there are residual stresses at the ends of the case which weaken the rod at that location. Hence it is desirable that such stresses be situated where the cross-sectional area or mass is greatest. The wrench square 13a of FIGURE 3 can be uniformly casehardened more readily; hence with this form I prefer to extend the case to include the wrench square and into the pin shoulder 14. The case hardness should be within the range of 400 to 800 Brinell or preferably 475 to 650.

Next I further compress the martensitic case (already in compression), preferably by shot-peening the rod surface. I may advantageously peen the rod while it is at a slightly elevated temperature (up to about 300 F.). When the rod subsequently cools to ambient temperature, the resulting thermal contraction adds to the compressive stress. This may also enhance the rate of dynamic strain aging and consequent pinning of dislocations. The resulting compressive stress may be of any magnitude up to the elastic limit of the steel, but should be at least 40,000 p.s.i. My preferred range is about 60,000 to 125,000 p.s.i.

According to an alternative procedure, I may simply water-quench the forged rod from the red heat at which it comes off the upset machine. The ends are unmachinable as water-quenched. As I heat the rod by induction, I apply sufficient heat to temper the rod. At the ends which are to be threaded, I apply heat by induction but I do not follow this by quenching, as along the remainder of the rod. I remove the soft skin portion at the ends before threading. The remainder of the treatment is as already described.

When the rod is in service, the stress pattern attained by my method lowers the net tensile stress, which is the main cause of fatigue cracking. The substantially martensitic case is necessary to furnish the desired stress pattern without expensive alloys or expensive treatment, such as nitriding or carburizing. The substantially martensitic case also improves the corrosion resistance of the rod, compared with a conventional rod which has a two-phase structure of iron carbide and ferrite. As already mentioned, the hardened surface of the rod might be expected to increase the sensitivity of the rod to hydrogen embrittlement, which would lower its ability to withstand tensile loads. My invention overcomes this difiiculty by supplying additional compressive stress at the rod surface. Thus when the rod operates within a Well, the surface either carries a net compressive load or only a negligible tensile load. Fracturing would originate in the rod surface, but no fractures begin in the absence of tensile loads to cause them. In a series of tests, I connected undersize rods or %1 inch diameter) of my invention in strings of larger conventional rods or Vs inch diameter). In normal operation the stress on the larger rods was on the order to 20,000 p.s.i., but was much greater on the smaller rods because of their smaller cross section. In every instance the larger rods failed first.

I recognize it is known to shot-peen sucker rods, but I believe it is novel to employ shot-peening in combination with induction-hardening confined to a critical portion of the rod surface. This portion has a case which is both hard and precompressed. The combination of treatments is necessary to obtain the advantages of my invention.

I claim:

1. A method of processing a steel sucker rod the carbon content of which is at least 0.15% and which has threaded pins at each end, said method comprising induction-hardening the rod apart from said pins to produce a substantially martensitic case of a depth about 3 to percent of the rod diameter and a hardness within the range of 400 to 800 Brinell, and further compressing said case to provide a stress of at least 40,000 p.s.i. up to the elastic limit of the steel.

2. A method as defined in claim 1 in which the case depth is 5 to 8 percent of the diameter, and the hardness of the case is within the range of 475 to 650 Brinell.

3. A method as defined in claim 2 in which the further compression is obtained by shot-peening.

4. A method as defined in claim 1 in which the rod includes an enlarged elevator button, a wrench square and a pin shoulder between its body and pin, and said case terminates at said elevator button.

5. A method as defined in claim 1 in which the rod includes an enlarged elevator button, a wrench square and a pin shoulder between its body and pin, and said case terminates at said shoulder.

6. A method as defined in claim 5 in which said wrench square has rounded edges to facilitate production of a uniform case.

7. A method as defined in claim 1 in which the rod is austentized, quenched and tempered before it is induction-hardened.

8. A method as defined in claim 1 in which the rod is forged and water-quenched from a red heat after forging, before the induction-hardening step and the ends of the rod are tempered during the induction-hardening step.

9. A steel sucker rod the carbon content of which is at least 0.15% and which has threaded pins at its ends, the body of said rod apart from the pins having a substantially martensitic case of a depth about 3 to 15 percent of the rod diameter and a hardness within the range of 400 to 800 Brinell, said case being further compressed to provide a stress of at least 40,000 psi. up to the elastic limit of the steel.

References Cited UNITED STATES PATENTS 1,690,612 11/1928 Anderson et a1. 14812.4 1,764,068 6/ I930 Crook 148-143 2,281,850 5/1942 McKinney 148-36 RICHAIRD O. DEAN, Primary Examiner U.S.Cl.X.R. 

